pgsql: nbtree: Allocate new pages in separate function.

nbtree: Allocate new pages in separate function.

Split nbtree's _bt_getbuf function is two: code that read locks or write
locks existing pages remains in _bt_getbuf, while code that deals with
allocating new pages is moved to a new, dedicated function called
_bt_allocbuf.  This simplifies most _bt_getbuf callers, since it is no
longer necessary for them to pass a heaprel argument.  Many of the
changes to nbtree from commit 61b313e4 can be reverted.  This minimizes
the divergence between HEAD/PostgreSQL 16 and earlier release branches.

_bt_allocbuf replaces the previous nbtree idiom of passing P_NEW to
_bt_getbuf.  There are only 3 affected call sites, all of which continue
to pass a heaprel for recovery conflict purposes.  Note that nbtree's
use of P_NEW was superficial; nbtree never actually relied on the P_NEW
code paths in bufmgr.c, so this change is strictly mechanical.

GiST already took the same approach; it has a dedicated function for
allocating new pages called gistNewBuffer().  That factor allowed commit
61b313e4 to make much more targeted changes to GiST.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/CAH2-Wz=8Z9qY58bjm_7TAHgtW6RzZ5Ke62q5emdCEy9BAzwhmg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d088ba5a5aa410d39b64f013e8433ad9eb3d17f1

Modified Files
--------------
contrib/amcheck/verify_nbtree.c            |  15 +-
src/backend/access/heap/heapam_handler.c   |   9 +-
src/backend/access/nbtree/nbtinsert.c      |  48 ++--
src/backend/access/nbtree/nbtpage.c        | 354 +++++++++++++++--------------
src/backend/access/nbtree/nbtree.c         |   4 +-
src/backend/access/nbtree/nbtsearch.c      |  48 ++--
src/backend/access/nbtree/nbtsort.c        |   2 +-
src/backend/access/nbtree/nbtutils.c       |   7 +-
src/backend/optimizer/util/plancat.c       |   2 +-
src/backend/utils/sort/tuplesortvariants.c |   5 +-
src/include/access/nbtree.h                |  26 ++-
src/include/utils/tuplesort.h              |   4 +-
12 files changed, 269 insertions(+), 255 deletions(-)